1. Field of the Invention
This invention pertains to an automatic `S` correction circuit for use with a resonant scan deflection circuit.
2. Description of the Prior Art
Large, relatively flat screen display devices, such as cathode-ray tubes (CRTs) utilize electromagnetic deflection circuits and deflection yokes for deflecting the electron beam being emitted from the electron gun before the beam reaches and bombards the phosphor screen. A relatively linear ramp or sawtooth signal current could be made to flow in the deflection coil for deflecting the electron beam. However, if this linear, sawtooth signal current does flow in the deflection coil of a conventional CRT, the amount of each incremental deflection of the electron beam as measured on the face, plate per each increment of the deflection coil current increases with respect to the angle of the beam to the tube axis. This results from the fact that the center of deflection of the electron beam does not coincide with the center of curvature of the faceplate. The deviation from linear relationship between current and beam position increases towards the periphery of the faceplate. This phenomenon is more significant in wide deflection angle, short cathode-ray tubes.
In order to prevent the above-mentioned disadvantage from occurring, a relatively non-linear sawtooth deflection signal current is generated and flows in the deflection coil. As a result, the amount of each incremental deflection of the electron beam as measured on the screen or faceplatc per each increment of deflection coil current remains approximately constant. FIG. 1 illustrates the non-linear sawtooth deflection signal current i.sub.D which must flow in the deflection coil in order to maintain a constant relationship between increases in the deflection of the electron beam and increases in deflection coil current. Note that portion A of the deflection waveform shown in FIG. 1 is roughly part of a sine wave and therefore satisfies the relationship EQU i.sub.D =I sin .omega.t (1)
where I is a constant based on the peak deflection current and .omega. is the angular frequency of the sine wave. The circuit for providing the required nonlinear waveform is known as an S-shaping circuit, wherein an S-shaping capacitor is connected in series with the deflection coil, for example as disclosed in U.S. Pat. No. 4,241,296. The capacitance of the S-shaping capacitor is carefully chosen, dependent upon the inductance of the deflection coil and the scan rate, in order to provide the optimum deflection current waveform for a particular application. However, there is a distinct disadvantage with respect to the use of such known deflection circuits in television monitors or graphic display units in which the capability of operating at different or continuously variable scan rates is desired, since the optimum S-shaping capacitance must be varied as the scan rate is changed.
According to the present invention there is provided an automatic `S` correction circuit for use with a resonant scan deflection circuit including deflection coil means, said correction circuit comprising:
S-shaping capacitor means having a terminal for connection to the deflection coil means, said capacitor means being of selectively variable effective capacitance;
detector means connected to said S-shaping capacitor means for detecting the peak voltage and the peak-to-peak voltage across the capacitor means; and
control means connected to the detector means and the S-shaping capacitor means and responsive to the detected values of said peak voltage and said peak-to-peak voltage to vary the effective capacitance of the capacitor means to maintain said peak voltage and said peak-to-peak voltage in a predetermined relationship.
In a preferred embodiment of the invention the effective capacitance of the S-shaping capacitor means is changed in order to maintain the ratio of the peak voltage to the peak-to-peak voltage within a relatively narrow predetermined range of values.